The present invention relates generally to integrated circuit chip package technology and, more particularly, to a unique manufacturing methodology for a leadframe semiconductor package adapted to electrically isolate the semiconductor packages to facilitate leadframe strip testing, and to increase design flexibility and integration in leadframe semiconductor packages.
Integrated circuit dies are conventionally enclosed in plastic packages that provide protection from hostile environments and enable electrical interconnection between the integrated circuit die and an underlying substrate such as a printed circuit board (PCB). The elements of such a package include a metal leadframe, an integrated circuit die, bonding material to attach the integrated circuit die to the leadframe, bond wires which electrically connect pads on the integrated circuit die to individual leads of the leadframe, and a hard plastic encapsulant material which covers the other components and forms the exterior of the package.
The leadframe is the central supporting structure of such a package. A portion of the leadframe is internal to the package (i.e., completely surrounded by the plastic encapsulant). Portions of the leads of the leadframe extend externally from the package or are partially exposed within the encapsulant material for use in electrically connecting the semiconductor package to another component. In certain semiconductor packages, a portion of the die pad of the leadframe also remains exposed within the exterior of the package for use as a heat sink.
For purposes of high-volume, low-cost production of semiconductor packages, a current industry practice is to etch or stamp a thin sheet of metal material to form a panel or strip which defines multiple lead frames. A single strip may be formed to include multiple arrays, with each such array including a multiplicity of leadframes in a particular pattern. In a typical semiconductor package manufacturing process, the integrated circuit dies are mounted and wire bonded to individual leadframes, with the encapsulant material then being applied to the strip so as to encapsulate the integrated circuit dies, bond wires, and portions of each of the leadframes in the above-described manner. The hardening of the encapsulant material facilitates the formation of a mold cap upon the leadframes.
Upon the hardening of the encapsulant material, the leadframes within the strip are cut apart or singulated for purposes of producing the individual semiconductor packages. Such singulation is typically accomplished via a saw singulation process. In this process, a saw blade is advanced along “saw streets” which extend in prescribed patterns between the leadframes as required to facilitate the separation of the leadframes from each other in the required manner. The advancement of the saw blade along the saw streets concurrently cuts the molded plastic mold cap, thus facilitating the formation of a molded plastic package body upon each of the separated leadframes.
It should be noted that the saw blade used in the saw singulation process cuts through copper (i.e., the metal material typically used to fabricate the strip) approximately ninety percent of the time. As a result, cutting through copper in this manner often disadvantageously results in the premature wear of the costly saw singulation blade.
Another drawback of the saw singulation process is that the same also typically results in the burring of the leads of the separated leadframes. Saw-generated burrs at the seating plan of each lead in the leadframe adversely affect solder mounting and joint reliability. In current semiconductor package fabrication methodologies, lead burrs are controlled by limiting the feed rate of the saw blade along the saw streets and by using specifically developed, high cost saw blades. However, as will be recognized, the use of the high cost saw blades is undesirable due to the resultant increase in production costs, with the reduced feed rates needed to control burring adversely affecting production speed, and thus efficiency.
When feasible, semiconductor package manufacturers improve singulation throughput by employing a gang cutting process. In the gang cutting process, an array of saw blades are spaced apart according to the spacing of the saw streets. Then, as cutting begins, the saw blades work simultaneously to cut along multiple saw streets at one time. Thus, several rows of leadframes are separated simultaneously, thereby increasing production speed. However, a single array of saw blades is typically usable for only one design of a multiple leadframe strip, and this unadaptability may inhibit semiconductor package design changes because of the costs associated with obtaining appropriate singulation tooling. Also, if one saw blade in the array becomes damaged or wears faster than the other blades in the array, the entire array must be replaced, thereby necessitating additional maintenance expenditures.
In order to eliminate the drawbacks of the gang cutting process, semiconductor package manufacturers have also employed manufacturing methodologies wherein the singulation process is accomplished using a wire saw cutter, allowing the cutting edge to span the entire saw street at one time. Though this particular singulation process improves manufacturing speeds and the need for frequent cutting tool replacement, it also possesses certain deficiencies which detract from its overall utility. More particularly, such process also requires that the saw street(s) along which the wire saw cutter(s) extend(s) be linear. Thus, such singulation process is not suited for use in conjunction with leadframe strips which include leadframes of differing configurations and require a series of staggered, non-linear cuts to complete the separation of the leadframes from each other. The use of the wire saw cutter(s) spanning the entire saw street(s) also does not completely eliminate the formation of sawing burrs on the leads. Moreover, each of the above-described singulation processes typically includes the cutting of the side rails of the leadframe strip which protrude outwardly from the mold cap. The sawing of these side rails weakens the leadframe strip in a manner making the same difficult to handle.
In certain applications, the wire saw cutter(s) described above cut(s) through only the metal of the leadframe strip along the saw street(s), leaving the mold cap intact. In this particular process, the wire saw cutter(s) is/are used to form one or more isolation cuts which effectively electrically isolate the semiconductor packages from each other, thus facilitating leadframe strip testing. However, in this particular process, the mold cap is normally subject to some degree of normal mold warpage which results in uneven saw depth in the isolation cut(s). This uneven saw depth gives rise to occurrences of cracking in the mold cap, and hence in the package bodies of the completely formed semiconductor packages.
The present invention addresses these particular deficiencies by providing a method wherein waterjet, laser or etching may be employed to electrically isolate semiconductor packages in a manner facilitating leadframe strip testing. The present invention also increases design flexibility and integration in leadframe semiconductor packages. These, and other advantages of the present invention, will be discussed in more detail below.